1. Field of the Invention
The present invention relates to a unidirectional condenser microphone unit, more particularly to a unidirectional condenser microphone unit that can collect sounds in a lower frequency range by proximity effect depending on situations, even in the case where a diaphragm is placed on a rear acoustic terminal side of the condenser microphone unit.
2. Description of the Related Art
Generally, a unidirectional condenser microphone exhibits proximity effect. The proximity effect is an effect of rise of output level in a low sound range when a sound source, such as speakers' mouth, becomes close to the microphone. Sound collection using the proximity effect is often performed because a bass-rich voice can be collected therewith.
The proximity effect, however, causes a change of a lower frequency response depending on a distance from a sound source, and therefore, the proximity effect is undesirable when a sound quality should not be changed depending on the distance.
In order to solve the above problem, employed is a unidirectional condenser microphone having a diaphragm on a rear acoustic terminal, as shown in FIG. 9.
A condenser microphone unit 50 shown in FIG. 9 includes an insulating base 9 made of an electric insulator such as synthetic resin or ceramics, and a first and a second condenser element 51 and 52 supported by opposite sides of the insulating base 9. The condenser elements 51 and 52 are identical except being disposed laterally symmetrically. The condenser elements 51 and 52 include diaphragms 15 and 16 stretched with a predetermined tension over supporting rings 13 and 14 which are integrally formed on peripheral areas of resonators 11 and 12 made of metal plates.
Further, the diaphragms 15 and 16 are disposed opposite to fixed electrodes 19 and 20 through spacer rings 17 and 18. And peripheral portions on both sides of the insulating base 9 and peripheral portions of the resonators 11 and 12 are integrally assembled by connection rings 21 and 22.
The diaphragms 15 and 16 use synthetic resin thin films having a metal-, preferably gold, evaporated films on one side thereof. The fixed electrodes 19 and 20 are made of perforated metal plate having a large number of sound holes (not shown). Additionally, electret dielectric films may be provided on the fixed electrodes 19 and 20.
The resonator 11 and 12 include acoustic terminal holes 11a and 12a for collecting sound waves. A communication hole 9a is provided at the center of the insulating base 9, and both ends of the communication hole 9a are covered with acoustic resistance materials 23 and 24.
Further, air chambers A1 and A2 are provided between the fixed electrodes 19, 20 and the insulating base 9 in order to acquire velocity components through acoustic resistance materials 23 and 24. In an example shown in FIG. 9, tapered members 25 and 26 are disposed on the both side faces of the insulating base 9. The tapered members 25, 26 form conical surfaces with the acoustic resistance materials 23 and 24 as apex parts and the peripheral portions of the insulating base 9 as peripheries, and the air chambers A1 and A2 function as acoustic transducers having a speaker-cone-like shape.
FIG. 10 shows an equivalent circuit for a condenser microphone unit 50. Let the first condenser element 51 be on a front side, then P1 denotes a sound source on a side of the acoustic terminal hole 11a, m0f denotes mass of the diaphragm 15, s0f denotes stiffness of the diaphragm 15, r0f denotes an acoustic resistance, and s1 denotes acoustic mass of the air chamber A1. And P2 denotes a sound source on a side of the acoustic terminal hole 12a, m0a denotes mass of the diaphragm 16, s0a denotes stiffness of the diaphragm 16, r0a denotes an acoustic resistance, and s2 denotes acoustic mass of the air chamber A2. A resultant acoustic resistance of the acoustic resistance materials 23 and 24 is shown as r1.
In the condenser microphone unit 50 of this structure, in the case where the sound source P1 is on a side of the first condenser microphone element 51, for example, sound waves P2 from the rear acoustic terminal 12a toward the first condenser element 51 comes through the diaphragm 16 (m0a, s0a, r0a) of the second condenser element 52.
Therefore, because sound waves in low frequency range do not enter the first condenser element 51 side, the condenser microphone unit 50 performs as being omnidirectional and the proximity effect is hardly obtained. That is, this configuration is preferable for keeping the sound quality unchanged even though a distance from a sound source is changed.
A condenser microphone unit of this configuration was disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2011-55062.
However, collection of bass-rich sound using the proximity effect is not achievable with the condenser microphone 50 shown in FIG. 9. Thus, depending on the situation, it is necessary to prepare other types of microphones with which the proximity effect is available.